In 1.29, you could support any rebel faction as long as they were listed as the "likely rebel faction" for a province.
In 1.30, only rebel factions that are progressing towards an uprising because their province is above 0 unrest can be supported.
I checked the patch notes and I see no mentions of this being changed.
Theres also some rather strange behaviour, if you support rebels and the unrest in their provinces drops to 0 before they spawn, they cant spawn. Nor do you get any notification at all when this occurs.
And if you occupy provinces, then start supporting rebels in those provinces, they wont spawn when the progress hits 100%. You dont get any notification when this happens either. When it hits 100%, they dont spawn and you can click "support rebels" again as if you are not supporting any rebels at all. They will only spawn if someone else controls at least 1 province that they can spawn in.
E.G. You occupy Venice's provinces with Dalmatia cores and then start supporting Dalmatian seperatists, progress hits 100% but the rebels never spawn and the game acts as if you are not supporting any rebels in Venice at all.